885 research outputs found
Mixed magnetic phases in (Ga,Mn)As epilayers
Two different ferromagnetic-paramagnetic transitions are detected in
(Ga,Mn)As/GaAs(001) epilayers from ac susceptibility measurements: transition
at a higher temperature results from (Ga,Mn)As cluster phases with [110]
uniaxial anisotropy and that at a lower temperature is associated with a
ferromagnetic (Ga,Mn)As matrix with cubic anisotropy. A change in the
magnetic easy axis from [100] to [110] with increasing temperature can be
explained by the reduced contribution of cubic anisotropy to the magnetic
properties above the transition temperature of the (Ga,Mn)As matrix
Effect of Ga irradiation on magnetic and magnetotransport properties in (Ga,Mn)As epilayers
We report on the magnetic and magnetotransport properties of ferromagnetic
semiconductor (Ga,Mn)As modified by Ga ion irradiation using focused ion
beam. A marked reduction in the conductivity and the Curie temperature is
induced after the irradiation. Furthermore, an enhanced negative
magnetoresistance (MR) and a change in the magnetization reversal process are
also demonstrated at 4 K. Raman scattering spectra indicate a decrease in the
concentration of hole carriers after the irradiation, and a possible origin of
the change in the magnetic properties is discussed
Ion Irradiation Control of Ferromagnetism in (Ga,Mn)As
We report on a promising approach to the artificial modification of
ferromagnetic properties in (Ga,Mn)As using a Ga focused ion beam (FIB)
technique. The ferromagnetic properties of (Ga,Mn)As such as magnetic
anisotropy and Curie temperature can be controlled using Ga ion
irradiation, originating from a change in hole concentration and the
corresponding systematic variation in exchange interaction between Mn spins.
This change in hole concentration is also verified using micro-Raman
spectroscopy. We envisage that this approach offers a means of modifying the
ferromagnetic properties of magnetic semiconductors on the micro- or nano-meter
scale.Comment: 4 pages, 4 figures, to appear in Jpn. J. Appl. Phys. (Part 2 Letters
Magnetic anisotropy switching in (Ga,Mn)As with increasing hole concentration
We study a possible mechanism of the switching of the magnetic easy axis as a
function of hole concentration in (Ga,Mn)As epilayers. In-plane uniaxial
magnetic anisotropy along [110] is found to exceed intrinsic cubic
magnetocrystalline anisotropy above a hole concentration of p = 1.5 * 10^21
cm^-3 at 4 K. This anisotropy switching can also be realized by post-growth
annealing, and the temperature-dependent ac susceptibility is significantly
changed with increasing annealing time. On the basis of our recent scenario
[Phys. Rev. Lett. 94, 147203 (2005); Phys. Rev. B 73, 155204 (2006).], we
deduce that the growth of highly hole-concentrated cluster regions with [110]
uniaxial anisotropy is likely the predominant cause of the enhancement in [110]
uniaxial anisotropy at the high hole concentration regime. We can clearly rule
out anisotropic lattice strain as a possible origin of the switching of the
magnetic anisotropy.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Dynamic relaxation of magnetic clusters in a ferromagnetic (Ga,Mn)As epilayer
A new scenario of the mechanism of intriguing ferromagnetic properties in
Mn-doped magnetic semiconductor (Ga,Mn)As is examined in detail. We find that
magnetic features seen in zero-field cooled and field cooled magnetizations are
not interpreted with a single domain model [Phys. Rev. Lett. 95, 217204
(2005)], and the magnetic relaxation, which is similar to that seen in magnetic
particles and granular systems, is becoming significant at temperatures above
the lower-temperature peak in the temperature dependence of ac susceptibility,
supporting the cluster/matrix model reported in our previous work [Phys. Rev.
Lett. 94, 147203 (2005)]. Cole-Cole analysis reveals that magnetic interactions
between such (Ga,Mn)As clusters are significant at temperatures below the
higher-temperature peak in the temperature dependent ac susceptibility. The
magnetizations of these films disappear above the temperature showing the
higher-temperature peak, which is generally referred to as the Curie
temperature. However, we suggest that these combined results are evidence that
the temperature is actually the blocking temperature of (Ga,Mn)As clusters with
a relatively high hole concentration compared to the (Ga,Mn)As matrix.Comment: 8 pages, 7 figures, to appear in Phys. Rev.
New Measurements of Orbital Period Change in Cygnus X-3
A nonlinear nature of the binary ephemeris of Cygnus X-3 indicates either a
change in the orbital period or an apsidal motion of the orbit. We have made
extended observations of Cygnus X-3 with the Pointed Proportional Counters
(PPCs) of the Indian X-ray Astronomy Experiment (IXAE) during 1999 July 3-13
and October 11-14. Using the data from these observations and the archival data
from ROSAT, ASCA, BeppoSAX and RXTE, we have extended the data base for this
source. Adding these new arrival time measurements to the published results, we
make a comparison between the various possibilities, (a) orbital decay due to
mass loss from the system, (b) mass transfer between the stars, and (c) apsidal
motion of the orbit due to gravitational interaction between the two
components. Orbital decay due to mass loss from the companion star seems to be
the most probable scenario.Comment: 7 pages, 4 figures, accepted for publication in A&
Area-Specific Regulation of Quiescent Neural Stem Cells by Notch3 in the Adult Mouse Subependymal Zone
In the adult mammalian brain, neural stem cells (NSCs) generate new neurons throughout the mammal's lifetime. The balance between quiescence and active cell division among NSCs is crucial in producing appropriate numbers of neurons while maintaining the stem cell pool for a long period. The Notch signaling pathway plays a central role in both maintaining quiescent NSCs (qNSCs) and promoting cell division of active NSCs (aNSCs), although no one knows how this pathway regulates these apparently opposite functions. Notch1 has been shown to promote proliferation of aNSCs without affecting qNSCs in the adult mouse subependymal zone (SEZ). In this study, we found that Notch3 is expressed to a higher extent in qNSCs than in aNSCs while Notch1 is preferentially expressed in aNSCs and transit-amplifying progenitors in the adult mouse SEZ. Furthermore, Notch3 is selectively expressed in the lateral and ventral walls of the SEZ. Knockdown of Notch3 in the lateral wall of the adult SEZ increased the division of NSCs. Moreover, deletion of the Notch3 gene resulted in significant reduction of qNSCs specifically in the lateral and ventral walls, compared with the medial and dorsal walls, of the lateral ventricles. Notch3 deletion also reduced the number of qNSCs activated after antimitotic cytosine β-D-arabinofuranoside (Ara-C) treatment. Importantly, Notch3 deletion preferentially reduced specific subtypes of newborn neurons in the olfactory bulb derived from the lateral walls of the SEZ. These results indicate that Notch isoforms differentially control the quiescent and proliferative steps of adult SEZ NSCs in a domain-specific manner.
SIGNIFICANCE STATEMENT In the adult mammalian brain, the subependymal zone (SEZ) of the lateral ventricles is the largest neurogenic niche, where neural stem cells (NSCs) generate neurons. In this study, we found that Notch3 plays an important role in the maintenance of quiescent NSCs (qNSCs), while Notch1 has been reported to act as a regulator of actively cycling NSCs. Furthermore, we found that Notch3 is specifically expressed in qNSCs located in the lateral and ventral walls of the lateral ventricles and regulates neuronal production of NSCs in a region-specific manner. Our results indicate that Notch3, by maintaining the quiescence of a subpopulation of NSCs, confers a region-specific heterogeneity among NSCs in the adult SEZ
An Action Spectrum for Light-induced Primordium Formation in a Basidiomycete, Favolus arcularius
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